If you ignore the C part its pretty much like any other OO class based language. You send messages to objects. The message causes a method to be activated. The method can access / set instance variables and send messages to other objects. Quite similar to Ruby, Python ... etc.
Cocoa is also much simpler than the mess that is building applications using HTML, CSS and the DOM. It was designed specifically for the purpose that is used for, unlike web applications.
I don't think it would be all that difficult to learn the language of objective-c, but there is always more to learn the just a language.
JS, HTML, CSS, require a brower, and a text-editor. I don't see many people building iPhone apps outside of xcode. And that stack seems to work well, but when a person who spends a lot of time with the browser model of development, try the xcode way of development, there can be a shock sometimes. Its a metaphore change, that can sometimes be hard to wrap your mind around.
I also think it goes both ways.
I would also argue that your premise that HTML, CSS, and JS were not designed for web applications, is invalid, because we can't just look at technologies for what they were suppose to be used for.
Plenty of stuff has been developed for one thing, but ended up doing another.
It wasn't used because it was well suited for the task. It was used in spite of its limitations because it doesn't require software installation. This made such a huge difference in uptake, that it didn't matter that the user experience was inferior in many other ways.
Cocoa is also much simpler than the mess that is building applications using HTML, CSS and the DOM. It was designed specifically for the purpose that is used for, unlike web applications.